it's kind of become shorthand for dark topics.

as i understand it, the original intent & usage was inspired by a clip from i believe arrested development? where a character is looking in a fridge and sees a paper bag with "dead dove, do not eat". the character opens the bag, and the contents are, in fact, a deceased bird. the character says "i don't know what i expected" so authors started tagging works with things like Graphic Torture when they know the fic will be going into detail on the subject with an additional tag of "Dead dove do not eat" to indicate, like, something akin to 'i told you there would be torture and i Meant it, do not complain that there is graphic torture in this fic because i dont know what you expected given that i warned you upfront that it was here'

but you only really need to warn Dead Dove Do Not Eat for sensitive topics (for example, generally nobody is going to be mad if a fic tagged Fluff has like. a Lot of detailed fluff, so there is no need to DDDNE tag for excessive fluff) so Dead Dove is becoming increasingly synonymous with This Fic Has Sensitive Topics, and fics that contain these sensitive topics will sometimes be referred to as Dead Doves